1. Field of the Invention
The present invention relates to a circuit for generating an OFF-state voltage. More particularly, the present invention relates to an OFF-state voltage generating circuit of a thin film transistor liquid crystal display (hereinafter referred to as a TFT LCD), capable of regulating the magnitude of the OFF-state voltage.
2. Description of the Prior Art
A TFT LCD utilizes TFTs as an element for switching individual pixels on and off. The switching element has ON-state and OFF-state characteristics. The ON-state characteristic of the TFT is determined by the voltage transmission rate from a data line to a pixel when the TFT is turned on. The OFF-state characteristic is determined by the voltage storing rate in the pixel during an OFF-state. In order to obtain a good ON-state characteristic, the ON current should be large. In order to obtain a good OFF-state characteristic, the OFF current should be small.
FIG. 1 is a graph illustrating the voltage versus current characteristic of a TFT. The ON current is defined as the current when the magnitude of the applied voltage is larger than a critical voltage V.sub.ON. The OFF current is defined as the current when the magnitude of the applied voltage is smaller than the voltage V.sub.ON. As shown in FIG. 1, the magnitude of the ON current increases from I.sub.ON as the voltage increases. The curve for the OFF current has a minimum value I.sub.OFF. The magnitude of the OFF current increases from I.sub.OFF as the OFF voltage increases from the value V.sub.OFF. When the magnitude of the OFF voltage is in the range between V.sub.OFF and V.sub.ON, the TFT has a non-optional shut-OFF characteristic.
FIG. 2 shows a conventional OFF-state voltage generating circuit. Diodes D3, D4, D5 and D6 are serially connected in a reverse biased direction to ground. One terminal of a capacitor C4 is connected to a node N1 between the diodes D4 and D5, and the other terminal receives an inverted common voltage V.sub.COMB. One terminal of a capacitor C5 is connected to an anode of the diode D6 and the other terminal receives a common voltage V.sub.COM.
If a power supply voltage is 5V and a threshold voltage of a diode is 0.75V, the common voltage V.sub.COM and the inverted common voltage V.sub.COMB alternate between 0V and 5V. An electrical potential at the second node N2, at which the diode D6 and the capacitor C5 are connected, alternates between -2V and -7V. As a result, the magnitude of the OFF-state voltage is fixed to two values.
The TFT characteristics are different from panel to panel. Thus, the magnitude of the OFF-state voltage requires adjustment to the TFT characteristics in order to obtain a good image quality. However, as described above, the conventional OFF-state voltage generating circuits cannot adjust the magnitude of the OFF state voltage.